Japanese Patent Application Laid-Open (JP-A) Nos. 2006-060683 and 2005-120662 disclose conventionally-known keyless entry systems for locking/unlocking a vehicle's door by driving a door lock mechanism of a vehicle, using a radio communication technique.
According to the technique of JP-A-2006-060683, communications are performed between a radio IC tag incorporated into a cell phone and a reader/writer installed into a vehicle, thereby operating the vehicle's door lock mechanism so as to lock and unlock the vehicle's door.
According to the technique of JP-A-2005-120662, a user presses a request switch provided on a vehicle's door handle, thereby activating a communication circuit including an antenna which is provided in the vehicle's door handle and also generating a magnetic field within a predetermined range from this antenna. When an electronic key exists within the range of the magnetic field, an ID signal is sent from the electronic key to the communication circuit. ID information from the electronic key received by the communication circuit is compared with a proper ID that is kept in the vehicle. When it is determined that the ID from the electronic key is proper, the door lock mechanism of the vehicle is operated so as to lock or unlock the vehicle's door.